1. Field of the Invention
The present invention relates to an image signal processing apparatus, and more particularly to an image signal processing apparatus which can correct a signal containing noise resulting from fault pixels of a solid-state imaging device.
2. Description of the Related Art
A solid-state imaging device having a charge-coupled device (CCD) or the like is a small, light, reliable and low-maintenance device, and is incorporated in various types of cameras. Recently, a solid-state imaging device has been developed, which is designed for use in a high-vision camera, and its practical use is now greatly expected.
A solid-state imaging device for use in a high-vision camera should comprise many pixels, have high sensitivity, and operate at high speed. More specifically, it is demanded that the device have 2,000,000 pixels arranged in 1,000 rows and 2,000 columns and read 2,000,000 pixel signals at frequency of 74.25 MHz. Complex technology must be implemented to read so many pixel signals at such a high speed. It is difficult to form an uniform layer of semiconductor crystal in a specific area on the substrate of a solid-state imaging device. In most cases, the crystal layer has defects. Heat is generated at each defect during the use of the imaging device. Hence, any pixel having a crystal defect has a greater charge than a pixel having no crystal defects, so that the fault pixel forms a larger dot than a normal pixel.
In the case where 1,000,000 pixels are arranged in a 2/3-inch image format, the size of each pixel is about 5 .mu.m.times.5 .mu.m. If a dust particle sticks to such a tiny pixel during the manufacture of the solid-state imaging device, it will make the pixel a fault one, however small it is. The signal output from any fault pixel will make a noise, which will inevitably degrade the quality of the image reproduced from the pixel signals.
Methods are available for processing signals thereby to compensate for such pixel fault, as is disclosed in Published Unexamined Japanese Patent Application 55-32270 and Published Examined Japanese Patent Application 2-7227. The technique of correcting pixel fault, disclosed in these publications, is to use a signal processing apparatus having a sample-and-hold circuit. The sample-and-hold circuit holds a sampling pulse at any position where pixel fault has developed, thereby to replace the signal output by the fault pixel with the signal output by the immediately preceding pixel. This technique is disadvantageous in terms of speed, noise, and the like. Since the sample-and-hold operation is performed during the period of a pixel signal, the pulses used for this operation enter the pixel signal output by any normal pixel, whereas no pulses enter into the signal output by any fault pixel. The signal the fault pixel outputs, therefore, differs in level from the signals the normal pixels output. This makes it impossible to correct pixel fault reliably. Further, the sample-and-hold circuit needs to be operated at an extremely high speed since the pulses it uses have a width of 2 ns. If any pulse has jitter, a noise will affect the waveform of the output signal, in amplitude direction.
The conventional signal processing apparatus cannot process signals at a sufficiently high speed since the sample-and-hold operation is performed during the period of a pixel signal, inevitably generating a noise. The circuit is also disadvantageous in that noise is generated as data is read from a ROM storing the data representing the position where pixel fault has been corrected. The fault-correcting circuit incorporated in the conventional signal processing apparatus corrects one pixel fault at one position, not able to correct two or more pixel faults at one position.